For the spectral sensitization of silver halides, spectral sensitizing dyes having an adsorptive property for silver halide (i.e., dyes acting as sensitizing dyes by adsorbing on the surface of silver halides) are generally used and spectral sensitization is attained by the injection of light-excited electrons from the dye adsorbed on the surface of the silver halide.
As such spectral sensitizing dyes, methine dyes which have an adsorptive property and a proper oxidation-reduction potential, such as cyanine dyes, merocyanine dyes, complex cyanine dyes, and complex merocyanine dyes are widely used. However, it is known that in spectral sensitization by these adsorptive dyes, there is a limit on the extent of the spectral sensitization attained since the amount adsorbed of the sensitizing dye on the surface of silver halide is limited, and that saturated adsorption of the dye or adsorption similar to the saturated adsorption frequently causes remarkable desensitization (dye desensitization). Thus, attempts to perform spectral sensitization with non-adsorbed dye molecule utilizing energy transfer from a dye molecule in the non-adsorbed state, to an adsorbed sensitizing dye molecule without the need for adsorption of the dye onto the surface of silver halide are disclosed, for example, in JP-A-51-117619, 62-239143, 63-138341 and 63-138342 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
In these attempts, after spectrally sensitizing silver halide grains by adsorptive dye(s) to optimum sensitivity, an energy transfer type dye is added to the binder at a high concentration to utilize the light-collecting effect of the energy transfer type dye, whereby an increase in spectral sensitization (hereinafter referred to as light-collecting sensitization) is attained.
In light-collecting sensitization, a high light-collecting sensitization effect is obtained in a system having a sufficiently high concentration of an energy transfer type dye (light-collecting dye) in the binder of a silver halide emulsion. Similarly, in regard to an adsorptive sensitizing dye which is an energy acceptor, the use of silver halide grains having a larger specific surface area and, hence, a larger amount of adsorbed sensitizing dye per emulsion grain, such as tabular silver halide grains, gives a higher light-collecting sensitization as disclosed in the example of JP-A-63-138342. In other words, a silver halide emulsion system having a larger amount of adsorbed spectral sensitizing dye per silver halide grain shows a more improved light-collecting sensitization effect.
However, in conventional light-collecting sensitization, since the light-collecting dye is of the non-adsorptive type for silver halide grains, it sometimes happens that in the system of a multilayer silver halide photographic material, in particular, a multilayer color photographic material, the light-collecting dye diffuses into other layer(s) than the layer in which the dye was incorporated to cause photographically undesirable effects in the diffused layer(s) such as desensitization by a filter effect, unnecessary spectral sensitization at a long wavelength region, color mixing, etc.
Also, in conventional light-collecting sensitization, energy is transmitted from the light-collecting dye to a spectral sensitizing dye adsorbed on the surface of the silver halide by energy transfer. However, it sometimes happens that if the distance between the light-collecting dye and the spectral sensitizing dye is increased by diffusion of the light-collecting dye, the energy transmitting effect is reduced to cause an effect similar to that of a filter dye in simply absorbing light contributing to the sensitization effect, whereby desensitization occurs instead of sensitization.